The invention relates to a system for inductive power transmission, which system comprises a primary unit having a primary coil and a secondary unit having a secondary coil. The primary coil generates a magnetic transmission field in a transmission area between the primary unit and the secondary unit.
Inductive power transmission systems are known from the state of the art. An induction charging system for a vehicle for charging an electric energy accumulator is described, for example, in French Patent Document FR 2 947 114 A1. The induction system consists of a ground unit having a primary coil and of a vehicle unit having a secondary coil. The distance between the two coils during the charging amounts to approximately 0.1 m.
An induction charging system for an electric vehicle is designed for the transmission of power in the range of several kilowatts.
It is an object of the invention to provide an improved system for the energy transmission.
This and other objects are achieved by a system for inductive power transmission, which system comprises a primary it having a primary coil and a secondary unit having a secondary coil. The primary coil generates a magnetic transmission field in a transmission area between the primary unit and the secondary unit. The system further comprises a metal detector, wherein the metal detector is suitable for detecting a metallic object situated in the transmission area.
According to the invention, the system comprises a metal detector which is suitable for the detection of a metallic object situated in the transmission area.
In the case of the inductive power transmission, for example, for charging an electric vehicle, field intensities therefore prevail in the transmission area that are so high that a metallic object experiences considerable heating as a result of induced eddy currents. It is advantageous to detect metallic objects in the transmission area or in the proximity of the transmission area early, i.e. before such a heating can occur.
According to a preferred embodiment of the invention, the metal detector has at least one pair of coils, which comprises a transmitter coil and a signal coil.
It is advantageous for the metal detector to be operable according to the pulse measuring method and/or the alternating-current measuring method by use of an alternating frequency.
This means that, in the transmission area, a magnetic field generated by the transmitter coil is superimposed on the transmission field. This magnetic field is called a detector field.
According to a particularly preferred embodiment, the signal coil has at least two coil elements, which are wound in opposite directions, are electrically connected in series and are symmetrical with respect to the axis along which the transmission field is oriented in the transmission area.
It can thereby be ensured that the signal coil is insensitive to the almost homogeneous transmission field. If the two symmetrical coil elements are penetrated by a homogeneous magnetic field, there will be no resulting total voltage because the voltages induced in the two coil elements are antipolar series. As a result, the transmission field is separated off in the reference system of the signal coil, and the signal coil is particularly sensitive to changes of the detector field because of metallic objects entering into or situated in the transmission area.
According to a further variant, the primary coil generates the transmission field with a transmission frequency that exceeds the alternating frequency multiple times or falls below it multiple times.
This results in an additional possibility of separating a residual induction of the signal coil by the transmission field from the induction by the detector field or the detector field disturbed by a metallic object during the signal detection.
An arrangement is particularly advantageous which has the system according to the invention for the inductive power transmission and has a vehicle to which electric power can be inductively transmitted, and which comprises the secondary unit that is situated in the area of the underbody of the vehicle, and where the primary unit is situated outside the vehicle and the metal detector is a part of by the primary unit or the secondary unit.
This means that the vehicle can be inductively charged, and the inductive charging system has a metal detector. The metal detector makes it possible to monitor the power transmission area between the primary unit and the secondary unit with respect to the presence or the penetration of metallic objects and, if required, influence or interrupt the charging operation. This results in a high degree of system safety because metallic objects could be heated by eddy currents to the point of self-ignition at the field intensities prevailing for charging a vehicle.
The invention is based on the considerations described in the following.
With respect to inductive systems for power transmission, for example, for charging a vehicle having an electrified drive train, there are currently no systems which detect metallic objects in the transmission path in a cost-effective and reliable manner. Optical systems, such as an infrared camera, can easily become dirty and detect a heating of the metallic object only if the metallic object is not covered by a neutral body. Appropriately dimensioned ultra-sound systems detect all objects that are sufficiently large compared to a vehicle. Small or flat objects (for example, coins) are not detected. Under certain circumstances, they may even reduce the availability because also non-metallic objects may possibly be detected as being metallic. Magnetic and metallic objects present in the system as well as the very intense transmission field interfere with a classic metal detector, known, for example, from geology, during the charging of, for example, an electric vehicle.
Inductive systems for power transmission have the disadvantage that they heat up metallic objects situated in the transmission path. This leads to risks, such as the danger of injury when touching the objects or because of the ignition of flammable substances in their proximity.
It is therefore suggested to modify a classic metal detector system with respect to some features in order to be able to utilize the common proven system also in the inductive transmission path and during the inductive power transmission.
Essential design elements for the functional separation of the detector function from the inductive energy transmission are the design of the detector coils as well as the utilization of a frequency range clearly remote in the frequency domain (detector to transmission) with a corresponding signal filtering.
Other objects, advantages and novel features of the present invention will become apparent from the Mowing detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.